160 CHEMICAL ENGINEERING
u3 to vz is greater than that of u2 to vz. This type of diffuser is generally made stationary and its sides are made radial, converging or diverging. With a slight convergence there can still be an increase in circumferential area with an increase in radius. When the sides diverge the angle of divergence should not exceed from 7 deg. to 10 deg., in order to conserve a full section of flow (see Fig. 18). A diffuser of this sort is also at times applied to a fan wheel running in a spiral casing, the air passing from the wheel to the diffuser and thence to the spiral, which must have small radial depth.
Characteristics of Various Designs and Effect of Change in Elements of Design and Proportions.—The effect of a change in a single element of design on the principal factors of fan performance is seen by comparison of the characteristic curves in Figs. 12 to 18. The curves are drawn, for fans handling standard air at a density of 0.075 Ib. per cubic foot, having wheels 1 ft. in diameter and operating at a peripheral speed of 4,000 ft. per minute or at 1,274 r.p.m. This facilitates comparison of one with another. The speed of 4,000 ft. per minute is not only a good average speed, but for standard air. represents the velocity producing a pressure of 1 in. of water gage. The diameter of 1 ft. was chosen in order to establish as a basis unit diameter and thereby simplify calculations. From these curves the performance at any other peripheral speed can be computed by considering the volume.to vary as the first power, the pressures as the second power and the horsepower as the third power of the speed, while the efficiency remains constant for any given load point on the curves. The performance of a symmetrical fan of different size or the computation of size, horsepower, revolutions per minute, etc., necessary to do a given work, is readily accomplished by use of the above rules and the further rule that the capacity of a given design of fan operating against a given static pressure at a given peripheral speed varies as the square of the diameter. All effective fan dimensions will vary as the first power and areas as the second power of the diameter. Where the fan handles air or gas at a density different from standard, the pressure and horsepower will vary directly with the density, and the mechanical efficiency will not be affected. For factors to apply with variations to the air (or gas) temperature, see Table 1, p. 143.
Figure 12 deals with a multiblade type of wheel fitted with a single-inlet spiral casing. The blades are of relatively small radial depth. The curves with solid lines are for blades having radially disposed corrugations equally spaced along the axial length of the blade; the broken-line curves are for similar blades without corrugations. They are curved so that the concave surface moves forward in rotation and are so inclined forward that a chord of the arc of the blade is about 18 deg. ahead of a radius through the tip of the blade, thereby making the outlet ports smaller than the inlet ports of the blades. The principal proportions, given as functions of the wheel diameter, are as follows: Number of blades, 60; radial depth of blades, 0.066D; axial length of blades, 0.52D; diameter at inlet of wheel, 0.868D; equation of spiral, R = r(l -f 0.198a); width of spiral, 0.692); number of inlets, 1; diameter of inlet cone, 1.05D; area of cutoff point, 0.434D2; area of outlet, 0.59D2.
Method of Using Characteristic Curves.—To illustrate the use of these characteristic curves a problem may be taken, as follows: What will be the size of a fan with corrugated blades of the type covered by Fig. 12 which will be required to deliver 50,000 cu. ft. per minute against 1.5 in. static pressure, the fan to operate at approximately maximum mechanical efficiency? The diameter of the fan found by calculation will vary according to the point on the curves of Fig. 12 taken as a basis, and will